Luminous Life 81
It’s 7 a.m., and the ROV is about to launch.
Men in hard hats scurry about, making final
checks. Then an enormous metal arm lifts the
ROV off the floor of the boat. Next the floor
where it had been sitting folds open, revealing
a square of ocean several feet below. The metal
arm lowers the ROV into the water; a moment
later, the vehicle disappears beneath the waves.
As a place to live, the ocean has a couple of
peculiarities. The first is that in most of it, there
is nowhere to hide. This means invisibility is at
a premium. The second odd thing is that as you
descend, the sunlight disappears. First red light
is absorbed. Then the yellow and green parts
of the spectrum disappear, leaving just the
blue. By 700 feet deep, the ocean has become a
kind of perpetual twilight, and by 2,000 feet, the
blue fades out too. This means that most of the
ocean is pitch-dark. All day, all night. Together
these factors make light uniquely useful as a
weapon—or a veil.
Consider the problem of invisibility. In the
upper layers of the ocean—the part where light
penetrates—any life-form that does not manage,
somehow, to blend in with the water is in dan-
ger of being spotted by a predator—especially a
predator swimming beneath, looking up.
To get a sense of this, imagine that you’re
scuba diving in the middle of the Pacific. Above
you, the place where the sea meets the sky looks
silver. Below you, the water shades into a dark
blue. In all other directions, it is a murky green-
ish gray. The seafloor, though you can’t see it, is
a vertiginous 11,000-plus feet below you. And
wait—what’s that shadow down there? Is it a
shark? All of a sudden you become aware of
how vulnerable you are: a great dark silhouette
against the silvery surface, visible to any hungry
animal that might be swimming about below.
Many life-forms solve this problem by not be-
ing there at all. They avoid the light zone during
the day, rising toward the surface only at night.
Many others solve it by evolving into transpar-
ent, ghosty creatures. On the dive, the first thing
you’d notice is that nearly all the life-forms you
meet, from jellyfish to swimming snails, are see-
through. In another approach, some fish—think
sardines—dissolve their silhouettes by having
silvery sides. The silver functions as a mirror
and allows the animal to blend in by reflecting
the water around it.
And some creatures—such as the shrimp
Sergestes similis, certain fish, and many squid—
use light. How? By illuminating their bellies so
as to match the light coming down from above.
This allows the animals to mask their silhouettes,
donning a kind of invisibility cloak. The cloak
can be turned on and off at will—and even has
a dimmer switch. S. similis, for example, can
alter how much light it gives off depending on
the brightness of the water around it. If a cloud
passes overhead, briefly blocking the light, the
shrimp will dim itself accordingly.
But if the aim is to remain invisible, why do so
many creatures, from ctenophores to dinoflagel-
lates, light up when they are touched or when the
water nearby is disturbed? A couple of reasons.
First, a sudden burst of light may startle a preda-
tor, giving the prey a chance to escape. A deep-
sea squid, for example, can give a big squirt of
light before darting off into the gloom. The green
bombers can throw their light grenades, and then
disappear into the darkness while the predator is
distracted by the light. The ctenophore can van-
ish while the predator lunges at its ghost.
Second, on the principle of the enemy of my
enemy is my friend, giving off light may serve to
summon the predator of the predator. Known as
the “burglar alarm” effect, this may be especially
important for tiny life-forms, such as dinoflagel-
lates, that cannot swim fast: For such extremely
small beings, water is too viscous to allow a quick
getaway. (It would be as if you were trying to
swim through molasses.) The chief defense for
these creatures is not fight or flight—but light.
Their flashes summon fish, which hang out in
the water, waiting. And when little shrimplike
critters (eaters of dinoflagellates) disturb the wa-
ter, causing the dinoflagellates to light up, the
Olivia Judson wrote on cassowaries in the September
2013 issue. David Liittschwager’s portraits of life-
forms appear frequently in National Geographic.